I'm well aware of what blue fin goes for in Japan.
Who's to say this tuna didn't go to Japan? Oh yeah, you did.
Processing Bluefin tuna once caught
Depending on where you are in the world and what you intend to do with your tuna, there’s a lot of conflicting advice with regards to how to kill, bleed, fillet and store your tuna. Fortunately there’s a few simple things to know and we’ll leave it up to you which techniques you want to employ.
Revive the tuna
These fish have a very high metabolic rate and when you catch them they are often exhausted when you finally get them to the surface. A build up of lactic acid during this time can spoil the quality of the meat if you bring your tuna aboard immediately and start bleeding it etc. For this reason, most of the higher quality commercial tuna fishermen will tie a rope around the tail and anchor this rope to the boat whilst also re-hooking the tuna in the mouth. Then they allow it to swim alongside the boat at a slow pace until the tuna is revived and the lactic acid build-up dissapates enough.
Fish that are not revived are known as ‘burnt’ or in Japanese ‘Yake’
Kill and Bleed the tuna
Some people prefer to spike the tuna by inserting a thin metal spike into the brain area located on the top of the head between the eyes. This is a very humane and quick way to kill the tuna however it also prevents the heart from beating faster than bleeding the fish out. Commercial fishermen tend to slide the arteries located just behind the pectoral fins and place the fish back into the water to bleed out over the next 10-15 minutes. This also helps cool the tuna while helping oxygenate the fish and preventing the issue mentioned above from occurring.
Which ever method you want to use, the main point here is that you want to remove as much circulating blood from the fish as you can without it pooling or coagulating around the meat areas. Excess and coagulated blood will taint the taste of the meat whilst also increasing the amount of undesirable bacteria in the flesh.
http://tunafishingcharters.com.au/processing-bluefin-tuna-caught/